Conventionally, drilling tubulars are transported in single lengths. A single length of tubular is for example 31 feet in length. Tubulars can however be used on a top drive-equipped drilling rig in triple lengths, that is, in lengths which are for example 93 feet long. Such combined lengths of tubulars are referred to as pipe stands. For efficiency during drilling, it is desirable to combine the single tubulars into for example triple length pipe stands off the critical path of the operation of the drilling rig so to not interfere with the drilling operation.
Moving pipe stands onto and off from the critical path of the drilling operation for tripping operations is often done with a robotic or at least power-assisted manipulating device, which at least in part replaces, or mechanizes or automates the manual function of members of the drilling rig crew referred to as derrickmen.
An associated part of the tripping operation is placing made up pipe stands into a storage rack mounted on the derrick mast. These racks are sometimes referred to as racking boards. Pipe stands must be manipulated into, and retrieved from the racking board during tripping operations. Numerous pipe racking systems have been developed and are available in the industry. For example, applicant is aware of pipe racking systems which are commercially available from National Oilfield Varco also referred to as NOV, Aker and Weatherford. All of these pipe racking systems are highly integrated, for example, integrated structurally, hydraulically, and/or electrically, into the drilling rig. Most of these are practical only for offshore applications.
Drilling rigs for use on land, referred to as land rigs, have additional mobility requirements, cost constraints and service access challenges. Consequently, in applicant's view, it is desirable to have a racking system for land rigs which is: (a) independently transportable, that is, transportable independently of the land rig; (b) relatively fast and easy to rig up and down; (c) applicable to a wide range of land rigs without significant customization or interface design, to enable manufacturing economies of scale and economy/flexibility of application, for example, temporary use, rental, shared use among rigs, provision as a separately contracted rig accessory; and (d) relatively easily and safely serviceable.
Numerous systems of which applicant is aware have been used for offline building of pipe stands, referred to as stand-building, wherein the stand-building is done either horizontally, or vertically, or for example built vertically in a mousehole. However, in the prior art stand-builders the following limitations or drawbacks are encountered: some stand-builders can only assemble doubles, that is, pipe stands having only two tubulars, some stand-builders require an unconventionally deep mousehole on every well, and also, mousehole-based stand-building systems require manual handling operations within close proximity of the rotating pipe at well center which is a safety concern; and some stand-builders cannot pass the assembled pipe stand directly to the top drive elevators, requiring an additional pipe stand transfer step.